HVAC
No heat: a cold-weather playbook
The short version
Start by protecting people and pipes, then check the thermostat, filter, breaker, and fuel supply. If you smell gas, hear a CO alarm, or see water leaking from the system, stop and call for help. Otherwise, work through the simple checks before you book a repair.
The furnace clicks on, hums for a minute, and quits. Or the thermostat is set to heat and nothing happens at all. Either way, the house has started a countdown, and the first question is not "what is broken?" It is "how fast is the indoor temperature falling, and who in this house cannot afford to get cold?"
Answer that before you open a panel or move a dial. A healthy adult in a well-insulated house has hours to troubleshoot. An infant, an older adult, someone with a medical condition, or a house that is already cool has much less time. The playbook below is written in that order: people first, then the simple checks, then the repair call.
Protect the people and the pipes
Pick one room to keep warm and close it off. Use layers, blankets, and safe supplemental heat. If the sun is out, open curtains on the south side and close them when it fades. Keep everyone off the cold floor if you can, and check on anyone who lives alone.
Pipes are the second clock. An unheated house in a hard freeze can freeze the supply lines within a day, sometimes faster in a crawlspace or against an exterior wall. Open the cabinet doors under sinks on exterior walls, let a few faucets drip, and keep the thermostat set to heat even if the system is struggling. The goal is to keep the house above freezing, not to make it comfortable.
If you use a space heater, plug it directly into a wall outlet, keep it three feet from anything that can burn, and never leave it running while you sleep or leave the house. Do not use the oven, a stovetop, or a charcoal grill for heat. A generator belongs outside, far from windows and doors.
The five-minute walk-through
Before you call anyone, run this list in order. It catches a surprising share of no-heat calls, and it gives the dispatcher useful information if you still need help.
- Thermostat: mode set to heat, setpoint above room temperature, fresh batteries, no blank screen.
- Filter: pull it and hold it up to a light. If you cannot see through it, replace it.
- Breaker: find the furnace or air-handler breaker and look for a tripped switch.
- Service switch: check the ordinary light switch near the furnace. It is often mistaken for a light.
- Fuel: confirm the gas valve is on, or that the oil or propane tank is not empty.
- Vents: make sure registers are open and not blocked by furniture, rugs, or curtains.
Thermostat checks
A thermostat can lose its settings after a power blink or a battery change. Confirm the mode is heat, not cool or fan-only. Raise the setpoint several degrees above the current room temperature and listen for a click. If the screen is blank, replace the batteries before you decide the thermostat is dead. If it is a smart thermostat, check the app for a schedule or a hold that is overriding your setting.
The filter is not a formality
A clogged filter restricts airflow. The furnace overheats, its safety switch shuts the burner down, and the house goes cold. The blower may keep running, which makes the system look alive while the heat is off. Replace the filter with the correct size and direction, then reset the system at the thermostat. If the filter was heavily loaded, expect the problem to repeat unless you change it more often.
Breaker and service switch
Look at the breaker panel with a flashlight, not your fingers. A tripped breaker sits between on and off. Turn it fully off, then fully on, once. If it trips again immediately, stop and call an electrician or the HVAC company. Do not tape it, prop it, or replace it with a larger size.
The service switch is a plain toggle near the furnace, often on the side of the unit or on a nearby wall. Someone may have flipped it while carrying boxes or cleaning. Check it before you assume the furnace is dead.
Fuel and pilot
If you have natural gas, make sure the shutoff valve at the furnace is parallel to the pipe. If you have propane, check the tank level. If you have oil, check the gauge and the filter. A pilot that will not stay lit is a job for a technician, and a gas smell is a job for the utility.
Match the symptom to the system
| What you notice | What it often means | What to do |
|---|---|---|
| Thermostat clicks, no fan, no heat | Power, thermostat, or control board | Check breaker and service switch, then call |
| Fan runs, air is cool | Burner not lighting, filter, or flame sensor | Replace filter, check fuel, call if it repeats |
| Short cycle, then lockout | Overheating, dirty filter, blocked vent | Replace filter, clear vents, call |
| Banging or popping at startup | Duct expansion or delayed ignition | Note when it happens, call for service |
| Water or rust around the unit | Condensate line or heat exchanger issue | Shut the system off, call |
| Hissing or a rotten-egg smell | Gas | Leave, call the utility or 911 from outside |
If you are already in it
The desk is open 7am–10pm ET, seven days a week. Call (877) 454-8068 and describe what is happening. The call is free; the independent professional sets their own charges.
Heat pumps need a different read
A heat pump in cold weather may run almost constantly and still deliver only warm, not hot, air. That is normal near its balance point. What is not normal is a unit that runs but blows air at room temperature, or one that is iced over across the top and down the sides. A light frost that clears during a defrost cycle is fine. A solid block of ice is not.
If the system has an emergency or auxiliary heat setting, use it only as the manufacturer intends. It is expensive to run and meant for short stretches, but it can keep the house safe while you wait for service. If the aux heat also fails, treat the house as unheated and move to the pipe protection steps.
Boilers and radiators
With a boiler, check the pressure gauge and the thermostat before anything else. Low pressure can stop the system; so can a thermostat that is calling for heat but not reaching the boiler. If some radiators are hot and others are cold, the cold ones may need bleeding. If the boiler is leaking, making a banging noise, or showing a fault code, shut it down and call.
Do not open the boiler jacket or adjust the pressure without knowing what the gauge should read. Steam boilers and hot-water boilers are different systems, and the wrong adjustment can be dangerous.
When the house is below safe temperatures
If the indoor temperature is falling toward the low 60s and you cannot hold it, stop troubleshooting and call. If it is falling below that with a vulnerable person in the home, treat it as an urgent situation and consider leaving for a warm place. A cold house is uncomfortable; a cold house with an infant, an older adult, or a medical device is a different problem.
Keep a thermometer in the room you are using so you are making decisions from a number, not a feeling. Check it every hour and write down the reading. That record is also useful when you call.
When to stop and call
Call the HVAC company when the system trips the breaker, repeats a lockout, leaks water, or fails to light after a filter change and a thermostat check. Call the gas utility or 911 first when you smell gas, hear a carbon monoxide alarm, or see smoke or flames. Do not relight a burner repeatedly, and do not stand in front of a unit that is banging or smoking.
If you rent, call the landlord or property manager before you authorize a repair. The rental maintenance guide explains how to document the problem and what to ask for.
What to tell the dispatcher
Have four things ready: the address, the type of system, what it is doing right now, and what you have already checked. "Gas furnace, thermostat clicks, blower runs, air is cool, filter was dirty and I replaced it" saves a round of questions. Mention any smell, noise, water, or fault code. Mention if anyone in the home is vulnerable.
Ask whether the visit includes a diagnostic fee and whether that fee is credited toward the repair. Ask what the after-hours rate is if you are calling in the evening. A clear answer now prevents a surprise later.
The bottom line
No heat is a house problem and a people problem at the same time. Protect the people, protect the pipes, and work through the small checks that do not require tools. If the system is leaking, tripping breakers, or making a smell you cannot identify, stop and call. The HVAC service page explains what a technician will look at, and freeze prep covers the pipe side of the same cold night.
A cold house rewards a calm first hour. Most of what you can safely do takes five minutes; the rest is knowing when to hand it over.